In a polyphase electric motor, three or more phase leads carry an alternating current (AC) voltage waveform at the same frequency and amplitude, but with a time offset or phase shift between the different phases. In a vehicle or other system using such a motor, an application of the AC voltage waveform to individual stator windings of the motor induces a changing magnetic field. This magnetic field causes the rotation of a rotor to occur, which in turn provides motor output torque. The motor output torque may be harnessed and directed as needed to perform useful work, e.g., propelling the vehicle.
In a vehicle transmission in particular, one or more electric traction motors may be used as part of a motor drive system, either alone or in conjunction with an internal combustion engine. Collectively, the prime movers provide input torque to the transmission. To ensure proper operation of the electric traction motor(s), a controller may conduct periodic measurements of phase currents and other electrical parameters.